There are many machining or fabricating operations requiring a workpiece of irregular shape to be held securely on a rotating support means, such that a portion of the workpiece is subjected to rotation about a predetermined axis of rotation. When the workpiece has a regular shape, such as is the case for cylindrical workpieces, there is no particular problem involved in holding the workpiece, for example, in the chucking headstock of a lathe or other machine tool, while effecting a facing operation, cutting a tread, or the like on the other end of the workpiece, or placing a bead of welding material around the periphery of the workpiece. However, when the workpiece has a shape other than a true cylindrical shape, such as, for example, when a workpiece is a pipe elbow, considerable difficulties are encountered in holding and centering the workpiece such as to cause the workpiece to rotate true around an axis of rotation, while being securely held on the end of a rotatable spindle such as a lathe headstock or the like.
A typical example of machining operation or fabricating operation in which conventional chucking fixtures are inappropriate is one involving holding securely a pipe elbow on the end of a rotatable spindle, for example, for facing, beveling or threading one end of the elbow or, more particularly, for welding on the end of the elbow a second elbow or a length of pipe by means of an arc or flame welder held in a fixed position, the two members to be connected being rotated in close proximity to the welder head.
As it is not possible to hold a pipe elbow, for example, by way of conventional chucking fixtures, such as conventional lathe headstock chucks, holding fixtures have been contemplated in the past consisting of an angle plate mounted in the lathe chuck at an appropriate eccentric position, and supporting the pipe elbow by tack welding the pipe elbow to the angle plate. Such an arrangement presents many inconveniences. The set-up must be changed for each size of pipe elbow. Tack welding of the elbow to the angle plate is a time-consuming and delicate operation which requires chiseling or grinding to separate the elbow from the angle plate at the end of the machining or fabricating operation. In addition, the surface of the elbow is marred, structural integrity of the workpiece is jeopardized and, when part specifications call for a particular metallurgical composition of the elbow material, such as for some stainless steel materials, contamination of the material may occur through transfer of material from the angle plate or due to the composition of the weld used for tack welding. Furthermore, nonmetallic pipe elbow cannot be readily held in the fixture by tack welding.
It is also readily apparent that the holding strength is dependent on the quality of the tack welding, and that it may happen that the tack weld breaks during a machining operation.